Flag message number which for deletion. On most servers
deletions are not actually performed until QUIT (the major exception is
Eudora QPOP, which deliberately violates the RFCs by doing pending
deletes on any disconnect).

Retrieves the message header plus howmuch lines of the message
after the header of message number which. Result is in form
(response, ['line', ...], octets).

The POP3 TOP command this method uses, unlike the RETR command,
doesn't set the message's seen flag; unfortunately, TOP is poorly
specified in the RFCs and is frequently broken in off-brand servers.
Test this method by hand against the POP3 servers you will use before
trusting it.

Return message digest (unique id) list.
If which is specified, result contains the unique id for that
message in the form 'responsemesgnumuid,
otherwise result is list (response, ['mesgnum uid', ...],
octets).